INVASION!
No, it's not Hollywood's latest thriller, but a real threat to our environment and economy. Increasing human
population and per capita consumption, leading to expansion of global trade, and increases in human mobility have
resulted in unprecedented invasion by nonnative species. These biological invasions produce severe, often
irreversible impacts on agriculture, recreation, and our natural resources. Invasive species threaten biodiversity,
habitat quality, and ecosystem function. They are the second-most important threat to native species, behind habitat
destruction, having contributed to the decline of 42% of U.S. endangered and threatened species. Introduced species
also present an ever-increasing threat to food and fiber production. In the United States, the economic costs of
nonnative species invasions reach billions of dollars each year.
What are Invasive Species?
Whether they are called invasive, nonnative, alien, exotic, or no indigenous, introduced species are those that
evolved elsewhere and have been purposely or accidentally relocated. While some species have invaded habitats on
their own(e.g., migrating wildlife, plants and animals rafting on floating debris), human exploration and colonization
have dramatically increased the diversity and scale of invasions by exotic species. Introduced species often find no
natural enemies in their new habitat and therefore spread easily and quickly.
How Did They Get Here?
Some nonnative species have been deliberately introduced in the U.S.-nonnative plants, like kudzu, were
introduced to control soil erosion; European birds were introduced to make colonists feel more at home; and game
fish have been widely transferred to stock sport fisheries. Some agricultural crops and trees have escaped plantations
and become pests, and the importation of agricultural and forestry products has brought assorted pest species and
diseases into new areas. Nonnative ornamental plants are used in landscaping around homes and businesses. Many
have moved into natural landscapes, some with significant ecological or economic impacts. Increased trade in
unusual pets introduces an increasing variety of species which may escape or be released by owners. The aquarium
trade is the source of a large number of aquatic species accidentally or intentionally released into waterways.
Aquaculture has also increased the spread of fish and other aquatic species and associated diseases. Huge metal
containers used in shipping goods via boat, train, and truck provide entry for seeds and small animals to new locales.
Thousands of species of marine organisms were and are moved around the world on ship bottoms, and hundreds
more have been moved globally by the wholesale transfer of edible oysters for "replanting." It is estimated that more
than 10,000 marine species each day may hitch rides around the globe in the ballast water of cargo ships (see
Aquatic Hitchhikers). Flooding can also transport nonnative aquatic and marsh species to new regions.
Human activities have improved the odds for many nonindigenous species. A rapidly increasing human
population has led to greater land disturbance which favors the spread of invasive species. Increased demand for
food and fiber and overuse of public lands for recreation and commercial purposes have also contributed to
nonnative invasions.
Aquatic Hitchhikers
When ships unload their cargo, they often fill their ballast tanks with water to provide balance for their return
journey. In addition to water, many aquatic organisms are sucked into these tanks and given transport. A ship will
then empty its ballast tank (and various aquatic stowaways) at the next port it takes on cargo. Many invasive species
have become introduced into new areas this way. A relatively simple control mechanism is to exchange ballast water
on the high seas between ports to remove invasive species before they reach the destination port. Other methods
being explored are using filters to trap organisms as the tanks are filled or heating the water to kill them.
The brown tree snake (Boiga irregularis), native to eastern Indonesia, the Solomon Islands, New Guinea, and
the northern and eastern coasts of Australia has caused ecological, economic, and human health problems on Guam
since it arrived shortly after World War II. It is an aggressive predator and has caused local extinctions of native
bird, bat, and lizard species.
Leafy spurge, (Euphorbia esula L.), native to Europe and Asia, infests almost 2.5 million acres of North
America. It has been reported to cause severe irritation of the mouth and digestive tract in cattle which may result in
death.
The Eurasian zebra mussel (Dreissena polymorpha) is a ballast-water stowaway. It is a voracious filter feeder
and removes suspended particles from the water, changing the physical characteristics of the invaded habitat and
causing native clams to starve to death. Colonies of zebra mussels attach to boats, pipes, and the shells of other
mollusks.
What Threats Do Invasive Species Impose?
Invasive species may negatively impact native species in any number of ways including: eating them,
competing with them, mating with them and decreasing genetic diversity, introducing pathogens and parasites that
sicken or kill them, and disrupting available nutrients. An introduced species can change the look and makeup of an
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entire system-changing species composition, decreasing rare species, and even changing or degrading the normal
functioning of the system. Maintaining intact natural systems is important to ensure the continuation of ecosystem
goods and services upon which humans depend.
Invasive plants can smother native vegetation or change the timing and severity of fires, floods, and other
disturbances. Introduced diseases and parasites can attack and eliminate dominant native plant species. For example,
the chestnut blight fungus from Asia all but wiped out the American Chestnut, thus changing the makeup of eastern
forests. Intentionally and accidentally introduced herbivorous mammals, such as goats, rabbits, pigs, and horses, can
threaten native (and often rare) plants by eating and trampling them. Often introduced herbivores diminish the
quality of the habitat for native species, for example by transforming shrublands and forests into grasslands.
In agricultural production, invasive plants outcompete crops for soil and water resources, reduce crop quality,
interfere with harvesting operations, and reduce land values. The U.S. Department of Agriculture estimates the
annual productivity loss of 64 crops due to exotic species at $7.4 billion. On rangelands, invasive plants, such as
cheatgrass, crowd out more desirable and nutritious forage, cause soil erosion, and poison some wildlife and
livestock species. In natural areas, nonnative plants, such as purple loosestrife, reduce habitat for native and
endangered species, degrade riparian areas, create fire hazards, and interfere with recreational activities. Aquatic
invasive species, such as the zebra mussel, clog lakes and waterways and adversely affect fisheries, public water
supplies, irrigation, water treatment systems, recreational activities, and shipping. In the ocean, nonnative marine
organisms are now impacting the availability of energy in regions such as the New England coast, Long Island
Sound, San Francisco Bay, and Hawaii. In San Francisco Bay, the Asian clam is now one of the species that by its
filtering regulates the energy flow in the Bay.
What Can Be Done?
The best way to limit impacts of nonnative species is to prevent them from invading and becoming established
in a new area. However, it is difficult to know if a species will become a problem in a given locale and we often
don't know why some species become invasive. Sometimes species don't pose problems until years after they've
been in an area.
Complete removal may be feasible early in an invasion or in a restricted area, but more often control or
containment is the only practical way to limit ecological or economic damage, especially for species that have
already invaded large areas. Control methods can include the following or a combination of all four:
Chemical control (using pesticides) can be effectively used to kill invasive species, but it can be problematic
due to impacts on non-target organisms (including humans), the development of resistance, and expense.
Mechanical control (physically removing the invasive species) is often successful, but can be expensive and
labor intensive.
Biological control (introducing a natural enemy-predator, parasite, or disease-often from the pest's native
range) can be an environmentally sound way to control invasive species with minimal expense, but some control
agents do not survive and others attack non-target organisms, i.e., become invasive themselves.
Ecological control (manipulating environmental factors such as fire and water flow) can provide native species
an edge in competing with invasive species.
Once the invasive species have been controlled or eliminated, ecosystems may need to be restored to return
lost components or functions to degraded areas. Restoring native communities is an important step to minimize the
chances an area will be reinvaded. It is also important to encourage activities that help keep lands and waters free
from invasive species. Because invasive species do not recognize property boundaries, successfully battling these
invasions requires partnerships between public and private landowners, government, industry, academia, and nongovernmental organizations.
Purple loosestrife (Lythrum salicaria L.), native to Europe, is a real problem on public lands. It is especially
suited for freshwater marshes, stream margins, and alluvial floodplains. It oftent invades rapidly, forming a
monoculture, and outcompetes native marsh plant species and affects bird and fish habitat.
Where Can I Get More Information?
Ecological Society of America, 1707 H St., NW, Suite 400, Washington, DC 20006. 202-833-8773. esahq@esa.org.
http://esa.sdsc.edu.
The Federal Interagency Committee for Management of Noxious and Exotic Weeds (FICMNEW). Co-chairs:
Deborah Hayes, Forest Service, 202-205-0847, deb@hayes-ent.com, and Gary Johnston, National Park Service,
202-208-5886, Gary_Johnston@nps.gov.http://refuges.fws.gov/FICMNEWFiles/FICMNEWHomePage.html.
Invasive Plants: Changing the Landscape of America. Factbook. (FICMNEW, Washington, DC: Summer 1998).
Contact: Sue Lauritzen, 318-266-8554, sue_lauritzen@usgs.gov.
Pulling Together: National Strategy for Invasive Plant Management (Washington, DC: U.S. Government Printing
Office, 2nd edition, 1998), http://bluegoose.arw.r9.fws.gov/ficmnewfiles/NatlweedStrategytoc.html.
The National Aquatic Nuisance Species Task Force. Nonindigenous Aquatic Species database, USGS, Florida
Carribean Science Center, Gainesville, FL. http://nas.nfrcg.gov/
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U.S. Department of the Interior, Office of Public Affairs, 1849 C Street, NW, Washington, D.C. 20240. 703-648422. http://www.doi.gov.
USGS Biological Resources Division, Office of Public Affairs. http://www.nbs.gov. paofficer@nbs.gov.
U.S. Fish and Wildlife Service, Office of Public Affairs. 202-208-4131. http://www.fws.gov.
Bureau of Land Management, http://www.blm.gov/weeds and http://www.blm.gov/education/weed/weed.html.
National Fish and Wildlife Foundation, 1120 Connecticut Avenue, NW, Suite 900, Washington, DC 20036. 202857-0166. http://www.nfwf.org. info@nfwf.org.
The Nature Conservancy, 1815 North Lynn Street, Arlington, VA 22209. 703-841-5300. http://www.tnc.org.
America's Least Wanted: Alien Species Invasions of U.S. Ecosystems (The Nature Conservancy, 1996).
http://www.consci.tnc.org/library/pubs/dd/.
Office of Technology Assessment, Harmful Non-Indigenous Species in the United States, OTA-F-565 (Washington,
DC: U.S. Government Printing Office, September 1993).
CONESQUENCES: Volume 2, Number 2 1996
Impacts of Introduced Species in the United States
by Daniel Simberloff
Since our country was first colonized, several thousand foreign plant and animal species have established
themselves in the U.S., as have hundreds of animal and plant disease agents, or pathogens. Many of these species-such as crop plants and some game animals--are beneficial, while others--such as most pets and ornamental plants-are harmless amenities. At the same time, some 15 percent of these interlopers have become invaders, causing
widespread problems that can prove serious and exceedingly costly. They can devastate farms and forests, impede
waterways, foul lakes and ponds, affect human health, and invade natural areas and replace native species.
Introduced species cause disasters that one would never have foreseen. It might not seem surprising that the
spread of fire-adapted, exotic plants that burn easily has increased the frequency and severity of fires, to the
detriment of property, human safety, and native plants and animals. But would one have guessed that, in 1936, the
town of Bandon, Oregon would be destroyed and eleven citizens killed by a fire propagated by gorse, a highly
flammable plant introduced, seventy years earlier, from Europe? Or that dense, flammable plants such as Australian
melaleuca, the so-called Australian pine, Asian cogongrass, and Brazilian pepper, introduced for roadside planting
in Florida, have become costly hazards because of water loss through increased transpiration, increased fires, and
blocked vision? The problem is so severe that the Florida Department of Transportation is removing 27,000
Australian pines along the Florida Turnpike.
The cost to taxpayers of introduced species in the U.S. was estimated, in a 1993 report of the Congressional
Office of Technology Assessment (OTA), to range from hundreds of millions through billions of dollars each year.
These estimates do not include effects on native ecosystems, such as extinction of native species that are of no
immediate economic concern.
Best documented are costs to agriculture: about a fourth of this country's agricultural gross national product is
lost each year to foreign plant pests and the costs of controlling them. In the case of cotton, the total accumulated
cost of the boll weevil, which arrived in the U.S. from Mexico in the 1890s, now exceeds 50 billion dollars. Leafy
spurge, an unpalatable European plant that has invaded western rangelands, caused losses of $110 million in 1990
alone. In eastern forests, losses to European gypsy moths in 1981 were $764 million, while the Asian strain that has
invaded the Pacific Northwest has already necessitated a $20 million eradication campaign. To keep U.S. waterways
clear of such plants as Sri Lankan hydrilla and Central American water hyacinth, about $100 million is spent
annually. The cost of Eurasian zebra mussels, which clog pipes in water systems such as cooling systems in power
plants, is predicted to be hundreds of millions of dollars annually.
Costs of introduced pathogens and parasites to human health and the health of economically important species
have never been comprehensively estimated, but must be enormous. A recent example is the Asian tiger mosquito,
introduced to the U.S. from Japan in the mid 1980s and now spreading in many regions, breeding largely in water
that collects in discarded tires. The species attacks more hosts than any other mosquito in the world, including many
mammals, birds, and reptiles. It can thus vector disease organisms from one species to another, including into
humans. Among these diseases are various forms of encephalitis, including the La Crosse variety, which infects
chipmunks and squirrels. It can also transmit yellow fever and dengue fever. The exotic disease brucellosis,
probably introduced into the U.S. in cattle, is now a major economic and ecological problem, for it causes
miscarriages in bison and elk as well as domestic livestock.
Introduced diseases can arrive through circuitous routes. When the North American rainbow trout was
introduced to Europe, for example, it suffered epidemics of whirling disease, caused by a parasite carried by the
European brown trout. Indiscriminate transport of rainbow trout from one breeding facility to another subsequently
spread the disease to other parts of the world, including back to America.
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ECOLOGICAL IMPACTS
Domination of ecosystems
Costs to natural systems, although not easily translated into dollars, are staggering and diverse. Perhaps the
greatest impacts are caused by plant species that come to dominate entire ecosystems. Melaleuca, which is
increasing its range in south Florida by some 35 acres each day, replaces cypress and other native plants and
provides poorer habitat for numerous animals. In many regions in and near the Everglades, it forms vast, dense
monocultures where no other plant can grow. Asian salt-cedar and Russian olive are introduced plants that have
formed new forests or replaced native plants along many rivers in the West, to the detriment of numerous bird
species. Red mangrove from Florida, planted in Oahu in 1902, has spread to fill many formerly-open coastal areas,
creating dense forests up to seventy-five feet high. Because each acre of mangrove swamp typically drops more than
four tons of leaves each year, and because their roots trap sediment and provide habitat for various marine animals,
this introduction has had enormous ecological impacts. Some of these may be positive, although most are as yet
unstudied.
Other damage to ecosystems
An introduced plant species can bring great changes to an entire ecosystem without dominating in either
numbers or biomass. On the island of Hawaii, the tall shrub Myrica faya, a native of islands in the eastern Atlantic,
has invaded young, nitrogen-poor lava flows and ash deposits on the slopes of Mauna Loa and Mauna Kea. Because
it fixes nitrogen, it modifies the normal colonization by other plants, favoring other introduced species.
Introduced animal species can also change the entire character of an ecosystem, through their effects on
vegetation and on which plants will survive. In the Great Smoky Mountains National Park, a population of wild, or
feral hogs, established by the few that escaped from hunting enclosures in 1920, has devastated local plant
communities--selectively feeding on plants with starchy bulbs, tubers, and rhizomes, and greatly changing the soil
by thinning the naturally heavy litter, and mixing organic and mineral layers together. The European periwinkle
snail, introduced to Nova Scotia as a source of food around 1840, spread southward, feeding on rhizomes of marsh
grasses and the algae that covered the rocks at the shoreline. In areas sheltered from waves, it was key to a gradual
transformation of the New England coast--into the bare rocks that most people interpret today as a natural condition.
Plant pathogens can equally overwhelm an entire ecosystem. The chestnut blight fungus arrived in New York
City in the late 19th century on nursery stock from Asia and in less than fifty years had spread over 225 million
acres of the eastern U.S., destroying virtually every chestnut tree. Because chestnut had comprised a quarter or more
of the canopy of tall trees in many forests, the effects on the entire ecosystem were staggering, although not all were
obvious. For example, several insect species that live only on the chestnut are now extinct or endangered. A less
predictable result is an increase in the oak wilt disease in many native oak species, since the trees that replaced many
of the chestnuts were red oaks, which are particularly susceptible to the blight.
Introduced species can drastically affect specific plants or animals even when the invaders do not modify the
entire ecosystem--feeding upon a native species, for example, to the point of its extinction. The semaphore cactus,
which was found until recently in the lower Florida Keys, is such a victim. Now a candidate endangered species, it
has all but disappeared as an indirect result of steps taken forty years ago to control another cactus that is also native
to the region. In 1957 the South American cactus moth was introduced to the island of Nevis in the Lesser Antilles
to control a pest cactus. Hopping from island to island, and possibly aided by inadvertent transport in cut flowers,
the moth reached the Florida Keys by 1990 and soon eliminated most of the semaphore cactus found in the wild.
Any that remain today must be protected by cages. Meanwhile, the cactus moth has found other host plants in the
Keys and has continued to grow in number.
Other examples abound. The balsam woolly adelgid, a European aphid, has killed almost all firs, once a
dominant tree at higher elevations of the Great Smoky Mountains National Park. The sea lamprey was able to
colonize the upper Great Lakes when the completion of the Welland Canal between Lake Ontario and Lake Erie
allowed this parasitic fish to bypass Niagara Falls and subsequently to work its way into lakes Huron, Michigan, and
Superior. It proceeded in ensuing years to devastate populations of lake trout, burbot, and whitefish, at great
economic loss to the region.
Disease
Introduced species can also propagate diseases that harm native species. In the Hawaiian islands, for example,
a battery of introduced Asian songbirds are host to avian pox and avian malaria. The diseases are transmitted to
native birds by introduced mosquitoes, and they have contributed greatly to an almost total elimination of native
birds, unique to Hawaii, in lowland forests.
Competition
Introduced species can compete with native ones. In Florida, the decline of the native wasp Pseudhomalopoda
prima --of value as a major enemy of a pest scale insect--corresponded closely with the arrival of another wasp from
southern Asia, Chrysomphalus aonidium, which had been introduced in 1990 for the same purpose. In many parts of
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the U.S., the European brown trout has been introduced as a game fish, and it is suspected of contributing to the
declining numbers of some native fish species, such as the golden trout of California.
Introduced plants can also harm native ones by producing and releasing chemicals. The African crystalline ice
plant, for example, has devastated native coastal vegetation in California. The ice plant is an annual that accumulates
salt, which leaches from its leaves when the plant dies at the end of the season. The salt, not surprisingly, suppresses
the growth and germination of native plants in these well-used but fragile habitats. And introduced animals can
simply attack native species, as does the South American fire ant, which arrived in Mobile, Alabama around 1940
and has since spread throughout the Southeast, replacing large parts of the native ant community.
Hybridization
Mating between some introduced and native species can lead to an extinction of the native species by replacing
some of its genes. For example, mallard ducks introduced to the Hawaiian islands for hunting have hybridized
extensively with the native, endangered Hawaiian duck, greatly complicating recovery plans for the latter species.
On the U.S. mainland, mallards migrate to Florida in the winter. Although they formerly bred only while in the
North, domesticated mallards released to the wild in Florida for hunting have bred with the native Florida mottled
duck, whose existence may now be threatened by hybridization. Rainbow trout introduced into western watersheds
as sport fish hybridize extensively with the Gila trout and the Apache trout--two species that are listed under the
Endangered Species Act. The native Pecos pupfish in Texas has hybridized so massively with the sheepshead
minnow, a widely used bait fish, that "pure" Pecos pupfish may no longer exist.
Plants can also fall prey to the same insidious phenomenon. An example is Lantana depressa, which is found
on a few dune and limestone ridge habitats of peninsular Florida. It hybridizes with Lantana camara, the descendant
of several Latin American or West Indian species that were brought to Europe as ornamentals in the seventeenth
century, hybridized by horticulturists, and then introduced by the late 18th century into the New World. And in
Texas, the Department of Transportation has hybridized the native white firewheel to extinction in its eagerness to
beautify roadsides with Indian blanket--a plant that is not found naturally in the range of the firewheel.
Even if no genes are exchanged between the hybridizing species, the process can threaten the existence of one
of them. Introduced brook trout are today displacing native bull trout in parts of the West. Although there is
extensive hybridization between the two species, the hybrid offspring are sterile, so they cannot transmit brook trout
genes back into the bull trout population. But the loss of productive mating opportunities by the rarer species, the
bull trout, contributes to its displacement.
At least three of the twenty-four known "extinctions" of species listed under the Endangered Species Act have
been wholly or partially caused by hybridization, and there seems to be no limit to other possible consequences.
Johnson grass was originally introduced to the U.S. around 1800 as a forage crop for cattle and is now viewed as one
of the worst weeds. Among other noxious traits, it hybridizes with cultivated sorghum to produce "shattercane,"
which is agriculturally worthless. Even worse outcomes are possible. For example, North American cordgrass,
which entered England in the holds of ships in ballast soil, hybridized there with an innocuous native species to
produce new plants that proved sterile. This might have ended the story, had not one of them undergone a mutation,
yielding a new species that turned out to be a fertile, invasive weed.
THE GEOGRAPHY OF INTRODUCED SPECIES
All parts of the U.S. now host troublesome introduced species, but not all states are affected equally.
Particularly hard hit are Hawaii and Florida, owing to their geographic location, mild climate, and their reliance on
tourism and international trade. Biologically, neither state is what it was 100 or 200 years ago.
In Florida, about 25 percent of many plant and animal groups are not native but introduced by humans in the
past 300 years, and millions of acres of land and water are dominated by invasive introduced species. In Hawaii,
about 45 percent of plant species and 25 to 100 percent of species in various animal groups were similarly
introduced. As a result, all parts of the Hawaiian islands except the upper slopes of mountains and a few protected
tracts of lowland forest are today dominated by introduced species. Other such regions of the world, including New
Zealand, are populated by similarly high fractions of introduced species.
Four features common to Florida and Hawaii account for their disproportionate number of introduced species.
The first is their geographic insularity: Hawaii is an archipelago of islands, while the Florida peninsula is a "habitat
island" bounded on three sides by water and on the fourth side by frost. A typical feature of islands is an
impoverished native flora and fauna, relative to mainland areas of equal size.
Second, in both Florida and Hawaii land cover is dominated by human-produced habitats, such as agricultural
and residential tracts. This is particularly so in the Hawaiian lowlands and throughout the southern part of Florida,
which are the regions most affected by introduced species. Land under heavy agricultural and residential use is often
unfavorable to native plant and animal species but is suitable for a variety of non- indigenous species.
Third, both states have large tropical or subtropical areas--particularly, once again, in the lowlands of Hawaii
and the southern part of Florida--and the absence of freezing temperatures allows many species to survive. It is no
accident that the northern limits of the ranges of many of Florida's introduced species are precisely where overnight
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freezes occur. Moreover, the tropical climate of the two states has encouraged the introduction and dissemination of
an immense number of non-native species that are sold as ornamental plants or pets. The accidental escape of an
exotic tropical ornamental such banana poka or Brazilian pepper, or of pets like bulbuls and tropical fish, would be
innocuous in most regions of the U.S., because of the local climate. In Florida and Hawaii, they thrive and spread.
The fourth distinguishing feature of these two states is that they are hubs for transportation and popular
destinations for tourists. Most visitors from Latin America enter through Miami, and many planes and ships from
other regions land there as well. Fully 85 percent of all plant shipments into the U.S. pass through Miami; in 1990
the total was 333 million plants--over 1000 for every man, woman, and child in the city, had they been asked to help
screen them. Hawaii is extremely popular with tourists and a center for both civilian and military traffic moving
through the Pacific.
Obviously, the likelihood of inadvertent introduction of alien species is enormous in both states. The upshot is
that in lowland Hawaii and southern Florida, large areas are now dominated by ecosystems comprised of species
introduced from all over the world, interacting in new ways and often facilitating one another's existence. For
example, several fig species imported into south Florida as ornamentals have become invasive because the wasps
that pollinate them have independently immigrated into the country, and the resulting fruits may be dispersed by
parrots introduced from various continents. Red-whiskered bulbuls from Asia roost in these trees and eat fruit of
exotic plants such as Brazilian pepper, loquat, and ornamental jasmines, contributing to the spread of these
introduced species. In the Hawaiian islands, feral pigs disperse many non-indigenous plants.
Other parts of the country are also under heavy assault by introduced species. Most of the sorts of problems
that have arisen in Florida and Hawaii have cropped up in other states, and the local and regional economic and
ecological costs may even exceed those of invaders in Florida and Hawaii. The invasion of leafy spurge in western
rangelands was cited earlier. Purple loosestrife, a weed from Eurasia, now covers wetlands in much of the
contiguous U.S. and is particularly troublesome in the Midwest, while the inter-mountain West has been radically
transformed by invasions of Eurasian annual herbs, such as cheatgrass, and the tendency of hoofed livestock to
interact synergistically with these plants. Although the percentage of introduced, exotic species in California is not
as high as in Florida and Hawaii, large portions of the state, including grasslands and many dune systems, are
dominated by exotic plants, and exotic fishes threaten many aquatic habitats.
Can the tide of invasions be stemmed?
STEMMING THE TIDE: KEEPING THEM OUT
The first line of defense against invasive exotics is not to allow them into the country in the first place.
Organisms have always moved around the globe, but the volume of traffic has increased continuously and
dramatically in the last three centuries with the increase in human travel and the movement of goods. There is no
real national policy to stem the tide, despite President Carter's Executive Order 11987 of 1977, which simply forbids
the import of any exotic species. Institutions and legislation to implement the order have not been forthcoming,
while a host of interest groups-- some private, some governmental--have fought to introduce more species.
Existing regulations
Under the Lacey Act of 1900, the U.S. Fish and Wildlife Service (FWS) is ordered to restrict the entry of fish
or wildlife that threaten humans, agriculture, horticulture, forestry, or wildlife. At present, however, the FWS views
very few species as posing such threats, and, in any event, a certain number of prohibited species are smuggled in.
The Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture prohibits the
introduction of several other vertebrate species because they may carry pathogens that affect poultry or livestock,
and the U.S. Public Health Service prohibits importation of other vertebrate species because they may carry
pathogens that threaten human health. But APHIS inspections are often perfunctory, as anyone who has entered the
continental U.S. on an airplane can readily confirm.
The Non-Indigenous Aquatic Nuisance Prevention and Control Act of 1990 authorizes FWS and the National
Oceanic and Atmospheric Administration to regulate introductions of aquatic nuisance species such as the zebra
mussel. Still, the Act has not been extensively implemented, and such wholesale introductions that occur, for
example, with the routine purging of ballast water from ships remain virtually unstemmed. Present proposals to
require at-sea (rather than in-port) pumping of ballast tanks would probably greatly lessen the influx of viable
individuals of introduced species. APHIS also has the authority to prohibit the entry of plants, under the Federal
Noxious Weed Act of 1974 and the 1939 Federal Seed Act, but it has been slow to exercise the authority and narrow
in its interpretation of "noxious." Such documented scourges as purple loosestrife, Brazilian pepper, and Eurasian
watermilfoil, for example, are not yet prohibited.
The troubles with introduced species do not always originate at the borders of the country: major problems
often ensue when a species from a state in which it is native moves to another in which it is not naturally found.
FWS has no authority to regulate interstate transport, beyond the enforcement of state laws that ban entry of specific
species. In any event, FWS is underfunded and understaffed, so that such regulation takes a very low priority.
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APHIS has the authority to regulate interstate movement but exercises it only to prevent the movement of pathogens
that might affect livestock or poultry.
Very generally, federal and most state agencies have adopted a policy of assuming that a species will pose no
problems unless proven otherwise and, therefore, have not demanded that individuals wishing to import a plant or
animal species demonstrate that its introduction will prove innocuous. Worse, many agencies actively promote the
import and spread of invasive exotic species, such as multiflora rose and Russian olive, without any serious
consideration of their potential consequences. Between 1935 and 1942 the U.S. Soil Conservation Service (SCS)
grew 85 million kudzu seedlings, promoted them for erosion control, and paid farmers to plant them, with a result
that has become legendary. State wildlife commissions fund expensive programs to import new game species with
little serious consideration given to the damage they might inflict. In general, interstate movement of species,
including fish and game animals, is viewed with even less concern than importation from outside the country. By
accident or design, funding for the enforcement of the meager regulations that now govern the movement of exotics
is woefully inadequate.
It is far more difficult to deal with introduced species once they are established than it is to keep them out, as
will be discussed below. Yet current federal laws are only invoked after a species is already in the U.S.--in other
words, too late. The 1993 OTA report and a recent paper by a team of biologists from the University of Washington
both propose far more stringent regulations on what importation should be allowed, pointing to a policy of "guilty
until proven innocent," rather than assuming that a species will be harmless. They further recommend that interstate
transport should be much more carefully regulated. Whether any of these laws or recommendations become a reality
must await the public will to do so, for a comprehensive approach to the problems caused by introduced species will
require sufficient means to ensure that the greatly tightened regulations are enforced.
ERADICATING THEM
Trying to eradicate every single individual of a harmful introduced species is a seductive but controversial
goal. One would like to eliminate ongoing, and sometimes increasing, damage, but the development of eradication
technology may prove daunting, and a failed attempt may be exceedingly costly and invoke colossal damage to nontarget species.
For example, the attempt to eradicate the introduced fire ant from southern states proved disastrous. In 1957, a
well- meaning Congress authorized $2.4 million for the project, but the initial heptachlor applications caused
wildlife and cattle deaths. Researchers next developed mirex bait, but the ant rapidly re-invaded areas from which it
had once been eliminated, and mirex residues were discovered in many non- target organisms. These findings
resulted in a ban by the Department of Interior on use of the insecticide on its lands. Registration of mirex was
finally canceled by the EPA in 1977, but by then the costs of the applications had climbed to about $200 million and
the range of fire ants had only expanded, dramatically, during the eradication campaign.
It is often said that eradication can be successful only if an unwanted species is detected early and while it lives
within a restricted area. Eradication campaigns in Florida against the Asian citrus blackfly seem to confirm this
view. The insect was found in Key West in 1934 and never left this island during a $200,000, three-year campaign
in which a mixture of paraffin oil, whale oil, soap, and water was widely sprayed. The last blackfly was seen three
years later, in 1937. The insularity of Key West was crucial, and other circumstances that greatly reduced traffic of
humans and commodities between Key West and the mainland prevented the insect from spreading. The same
species was discovered in 1976 in a much larger area centered on Fort Lauderdale. An eradication campaign was
again mounted, but the area was too large and low-level infestations recurred. In 1979 the eradication effort was
abandoned in favor of a more modest program of maintenance control, or "containment."
The case of the Giant Snail
The giant African snail, Achatina fulica, about three inches long, has been introduced widely in Asia, to
islands in the Pacific and Indian Oceans, and recently to the West Indies. It is seen as a serious agricultural pest, and
predatory snails such as Euglandina rosea that were introduced to attack it have only added to the problem by
extinguishing many native snail species. It was, however, eradicated in Florida--although neither easily nor cheaply.
In 1966, a boy returning from Hawaii smuggled three of the snails into Miami, and his grandmother released
them in her garden. Reproduction ensued, and in 1969 the Florida Division of Plant Industry (DPI) was alerted,
leading to an immediate survey. The state Commissioner of Agriculture notified the news media about the giant
snail, mailed over 150,000 copies of an attractive brochure, and called for public assistance in reporting and
eliminating it. An area covering about forty-two city blocks was quarantined, but within days, a second infestation
was discovered--in Hollywood, 25 miles north of Miami and well outside the initial quarantine zone.
The ensuing eradication campaign relied primarily on hand-picking, plus a granulated chemical bait. There
were frequent surveys, and by 1971 in a six months period only forty-six snails were found-- compared to 17,000 in
the previous sixteen months. In Hollywood, seventeen months after its initial infestation, only one adult snail was
found. But less than a month after the effort seemed to have succeeded, a third infestation, probably three years old,
was discovered three miles south of the original Miami site, with over 1,000 live snails on one block. The block was
quarantined, and a large buffer zone was surveyed and treated. Nine months later, a fourth infestation, again about
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three years old, was found two miles north of the original one, followed by a fifth, about half a mile north of the
initial infestation.
Although profoundly disappointed, the DPI persisted. By 1973, seven years after the three snails were brought
into the city, more than 18,000 had been found, and many eggs. In the first half of that year, by contrast, only three
snails were collected, in two sites. By April of 1975, no live specimens had been found for almost two years, and the
campaign--which had cost over $1,000,000--was judged successful. Frequent surveys were continued for many
months, along with the application of bait and chemical drenching. As a result, the giant African snail has not been
found again, anywhere in the state.
Other attempts
Even widely distributed pest species have been eradicated. For example, the citrus canker, a bacterium, arrived
in the Gulf states around 1910. By 1914 it had spread as far as Texas to attack grapefruit, orange, and other citrus
trees. A strict quarantine, chemical controls, and a major educational effort to keep citrus workers from spreading
the bacterium led in time to total eradication. By 1916, it had been reduced to mostly local outbreaks, and the last
infestation was seen about thirty years later. No new infestations were found through 1984, when the effort to
monitor the bacterium was effectively ended.
It is possible that new technologies of gene manipulation--in either a pest species or an organism that might
attack it--could lead to the eradication of introduced pests, including those that seem immune to more traditional
methods. However promising, the reliance of such genetic engineering projects on venture capital--plus the
economic reality that, if such a method worked, there would be no continuing market for it--militate against this
method, at least as genetic engineering is currently practiced in the U.S.. In Australia, where a somewhat different
approach is taken, the national government now sponsors the genetic development of organisms to eradicate
introduced rabbits and foxes.
Finally, eradication campaigns are occasionally beset by public disagreements over either the desirability of
eliminating an introduced species or the proposed methods. The attempt by the state of California in the 1980s to
remove exotic blue gum eucalyptus trees from Angel Island, a state park in San Francisco Bay, led to vociferous
organized protest by those who found the trees attractive. The opponents were either unoffended by the fact that the
species was an alien or unconvinced that, as the state had shown, the blue gum eucalyptus was a threat to native
species. Attempts in recent years to eradicate non-native mountain goats from Olympic National Park in Washington
and feral pigs from Nature Conservancy land in Hawaii have run into stiff opposition from animal rights activists-again, in the face of evidence that the introduced animals were harming natural ecosystems. Proposals to remove
feral horses and burros in California and elsewhere have sparked similar responses.
CONTROLLING THEM
If eradication is impossible, the population of an introduced species can often be controlled in order to contain
the level of economic and/or ecological damage. Such methods are called "maintenance control."
Pesticides
Chemicals, both insecticides and herbicides, are widely used for this purpose, although they must be carefully
chosen. The massive detrimental effects to the environment and to human health of broad-spectrum pesticides such
as DDT are widely known. Some of the newer pesticides, however, have far fewer side effects. For example,
glyphosate-based herbicides have been used with some success to control melaleuca and Brazilian pepper in south
Florida and cheatgrass in the West. The major drawbacks are their higher cost, the necessity of repeated application,
and the fact that they affect plants other than the pest species for which they are intended.
Similarly, insecticides are now available that, when used properly, pose no hazards to human health. Some of
these, such as those based on insect hormones, can minimize damage to non-target species. As with herbicides, the
newer insecticides are expensive. Moreover, a potential problem with both herbicides and insecticides lies in the
ability of targeted species to develop resistance against them. The pest can evolve physiological or behavioral
modifications that remove some or all of the intended impact. For example, on Long Island, New York, the
Colorado potato beetle has developed resistance to all major classes of insecticides.
Mechanical methods
Introduced pests can often be controlled mechanically. Water hyacinth, introduced from South America to
Florida at the end of the 19th century, spread to cover more than 120,000 acres of public waters by 1960, smothering
beds of submersed native vegetation and producing many other detrimental ecological effects. Coverage today is
maintained at around 3,000 acres by a combination of mechanical harvesting and treatment with the herbicide 2,4-D.
The key to this successful effort in maintenance control is to attack the plant continually and not to wait until
particular infestations reach problem levels. A key component of a Nature Conservancy maintenance control project
for yellow bush lupine--an invasive nitrogen-fixing species introduced to a northern California dune preserve--is
periodic "bush bashes," in which supervised public volunteers rip out as many lupine bushes as they can find.
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Biological control
Perhaps the major method of maintaining acceptable levels of introduced pest species is biological control, in
which a natural enemy, typically from the intruder's native region, is introduced to keep the pest under control. The
approach is a venerable one, for we know that green weaver ants were used by citrus growers in China to control
caterpillars and beetles as early as AD 900. A strikingly successful American project in 1889 entailed importing the
predatory Australian vedalia beetle to control the Australian cottony-cushion scale--a bug that was then devastating
California citrus groves.
Some introduced pest plants can also be controlled at acceptable levels by introduced natural enemies. St.
Johnswort, also known as Klamath weed, a native of Europe, Asia, and northern Africa, was introduced to
Pennsylvania in 1793 and by 1900 had spread to California. By the mid-1940s, about five million acres of rangeland
in North America had been rendered worthless for grazing. Two beetles, one from Eurasia, the other from Europe
and northern Africa, were released at that time, and within ten years St. Johnswort had become an unimportant
roadside weed, less than 1 percent as abundant as in the peak years of the 1940s. Microbial pathogens are also used.
For example, Bacillus popilliae now plays a key role in controlling Japanese beetles in the eastern U.S.
Roughly one in five of all recent biological control projects have led to economically significant control of the
target pest, and with virtually no continuing expenses. Yearly benefits from biological control programs in the U.S.
are estimated to exceed $180 million. Still, the method is not without its dangers. An imported predator, herbivore,
or parasitoid could potentially attack a non-target species, and several instances are known, particularly in earlier
projects, in which biological control agents caused major damage.
Most examples of biological control projects gone awry involve the introduction of predators, often
vertebrates, that attack a variety of prey species. The small Indian mongoose, for example, was introduced to the
Hawaiian islands in 1883-1885 to control introduced rats in sugarcane fields. Its taste for prey of all kinds
contributed to the decline of native birds in the Hawaiian archipelago, and especially ground-nesting species. The
predatory snail Euglandina rosea, introduced to the Hawaiian and many other Pacific islands in a futile attempt to
control the giant African snail, has devastated populations of several native Hawaiian snails. The mosquito fish
(Gambusia affinis and G. holbrooki ) has been introduced worldwide for mosquito control, and through predation
has brought about the decline of several native fish species. In Texas, mosquito fish have hybridized with a
restricted native species and now threaten its genetic integrity.
The hazards in employing biological control are like those of fighting fire with fire. Biological control agents
are, by choice, introduced species, and in early attempts, particularly, many of their ensuing ecological impacts
would have been difficult to predict. Moreover, an introduced biological control cannot simply be recalled if it is
found to have an unintended impact, as one can simply stop disseminating a chemical pesticide.
Today the professional biological control community has far more rigorous safety standards, especially for
agents proposed to control pest plants. There is often extensive laboratory testing of the palatability of non-target
species, such that generalized predators like the mongoose or Euglandina rosea would probably never be endorsed
by the professionals who today deal in biological control. One that slipped by, more recently, is grass carp, which is
often released for biological control of introduced aquatic weeds. This Asian fish devours native vegetation as well,
to the detriment of native aquatic communities. Recently devised techniques allow the grass carp that are introduced
to be genetically altered, as eggs, so that the fish are largely sterile. Controlling their subsequent reproduction in this
way may lessen the potential damage from this generalized herbivore.
Nonetheless, there may still be major inherent risks in biological control. For one, there is often little
monitoring after the release of a biological control agent to see just what it is doing, and this is particularly true
when the introduced species feeds on insects. Insect species are not routinely monitored, and it would be difficult to
know if a beneficial non- target insect was harmfully affected--particularly if it is far from the site of release of a
parasitoid or predator. Second, biological controls, because they are living organisms with built-in mechanisms to
disperse, can end up far from the intended theater of operations. The appearance of the cactus moth in the Florida
Keys, more than thirty years after its release in the Lesser Antilles, is a good example. Lastly, biological control
agents are sometimes released by groups or individuals acting on their own, without the benefit of professional
counsel or review. Publicity regarding biological control can easily inspire such action. In Florida, homeowner
associations and individual citizens have released the goldenhorn marisa snail in attempts to control hydrilla and
water hyacinth infestations, even though the marisa feeds indiscriminately on many desirable native plant species
and harms aquatic animals that use these plants as habitats.
CONCLUDING THOUGHTS
Introduced plants, animals, and pathogens often pose an initially hidden but eventually monumental problem in
the U.S.--or any other country whose doors are opened to traffic from other lands. Some costly invaders remained
inconspicuous in this country for decades, and then spread swiftly. Their harmful effects are often subtle and
surreptitious, but the eventual impacts on the economy or natural environment are no less real, and often disastrous
and even irreversible, as when native species disappear.
On a general level, introduced species often interact with the destruction and fragmentation of habitat, the
other major cause of our national conservation crisis. Areas of land that have been cleared are often colonized not by
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the plants that were removed but by introduced species. Moreover, in what is now a typical landscape of forested
patches interspersed with farms and residential areas, the forests are much more easily invaded by exotic plants and
animals than was the case when trees alone prevailed. Another major form of habitat destruction occurs when fireprone introduced plants foster catastrophic fires that eliminate native communities of plants and animals.
The staggering economic implications of this tide of invaders are often unappreciated. For example, in the
western rangelands alone, more than 16 million acres are heavily affected by alien plants--at great cost to cattleraising--and 2,300 additional acres are invaded each day. The alien species are consistently much less nutritious than
the native ones, if not totally unpalatable. Many of the ecological impacts cannot easily be given dollar values, but
they are likely to be enormous. Of no inconsiderable value are the impacts on native songbird populations that have
followed the introduction of the English or house sparrow in 1850, the starling in 1890, and the house finch in 1940.
Of the 632 species and subspecies recently officially listed as "endangered" under the Endangered Species Act,
forty-five are primarily threatened by competition from introduced species. The key threats to approximately 100
species on the list are introduced species that prey or feed on them. Five are threatened chiefly by exotic diseases
carried by introduced species. For 424 of the 632, habitat destruction is the main threat, but the nature of the threat
for many of the plants that are included is the domination of their new habitat by introduced species. And a
contributing threat for several of those listed as endangered is hybridization with introduced plants or animals.
Indeed, the great majority of introduced species do not cause problems of any sort. Most ornamental plants do
not establish themselves outside gardens, and most species of discarded or escaped pets cannot survive in the wild.
Of the minority of introduced species that do live for long outside human- dominated habitats, many are not
invasive.
What is important is that some introduced species do create problems--a fraction that has been estimated as
one in every seven that enter the country as aliens. And as we have seen, some of those problems are costly indeed.
One would like to be able to declare in advance which introductions will be innocuous and which will prove
disastrous. But ecologists and evolutionists, despite intensive research, are not always able to do this. Sometimes
they can spot a potential problem quite accurately. At other times, they can only rationalize, post facto, the observed
impacts, and learn from them. And they can occasionally point to classes of organisms--such as insects that attack a
wide variety of plants--that have a particularly high probability of proving problematic. But there will always be
exceptions to rules so generally drawn, and surprises, as from exotic plants or animals where the risk would have
been seen as quite low.
Because of these inevitable uncertainties, it seems unconscionable that the introduction of species and their
interstate transport are so minimally regulated. The complexity of ecological interactions, the history of past
introductions, and the potential ecological and economic costs of new ones strongly support the recommendations
that every proposed introduction be viewed as potentially problematic until substantial research suggests otherwise.
This reversal of the present policy--or lack of one--would replace the present operating tool of "blacklists" of
prohibited species with an approved "whitelist" of those species that have been studied intensively enough to allow a
reasonable judgment that no harm is likely. By this more prudent method of control, only species on the whitelist
could be introduced. There would still be occasional problems--whitelisted species may turn out to cause problems.
But the number of problem introductions would be far lower than today.
The patchwork arrangement of federal and state laws and agencies that now regulate introductions of plants
and animals came into being, over time, in response to isolated alarms and is wholly inadequate to address what is
now known to be a broader and more pervasive problem. It is no match for the heavy commerce in today's smaller
and ever more connected world. Needed is the designation of a single, lead agency, with a comprehensive mission
and with overall regulatory authority, if we are truly to confront this biological invasion in a meaningful way. Plants,
vertebrates, invertebrates, and pathogens all interact in synergistic and complex ways, and any alien that is released
into the natural environment becomes a part of this interconnected web. We cannot hope to address the potential
problems of any introduced species when, as is the case today, one organization regulates only certain plants,
another monitors a limited set of microbes, another is in charge of selected game animals, and yet another governs
mollusks.
-------------------------------------------------------------------------------For Further Reading
Biological Invasions: A Global Perspective. Edited by J. A. Drake, H. A. Mooney, F. di Castri, R. H. Groves, F. J.
Kruger, M. Rejmanek, and M. Williamson. Wiley, Chichester, U.K., 1989.
Biological Pollution: the Control and Impact of Invasive Exotic Species. Edited by B. N. McKnight. Indiana
Academy of Science, Indianapolis, 1993.
Eradication of Exotic Pests. Edited by D. L. Dahlsten and R. Garcia. Yale University Press, New Haven, 1989.
Harmful Non-Indigenous Species in the United States. U.S. Congress, Office of Technology Assessment. U.S.
Government Printing Office, Washington, D.C., 1993.
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